Pluto’s Flyby Data Finally Enters the Scientific Record
Pluto was supposed to be a dead little world. Too far from the sun to have active geology, and too close to the Kuiper Belt to be covered in anything but craters. Then, in July, the New Horizons probe flew by. And bit after bit, pixel after pixel, its data confirmed that this planet was colorful and icy, with mountains and glaciers, spewing gases, blowing winds, and blue skies. Pluto isn’t dead: It’s awesome.
Now Pluto’s awesomeness is officially part of the scientific record. Today—which happens to be the three month plus one day anniversary of the flyby—the journal Science published the first data from that historic occasion. On the surface, it’s a New Horizons highlight reel. But beyond being a scientific nostalgia-gram, the paper ties together all the geophysical, atmospheric, and surface composition data from the probe, presenting Pluto, Charon, Hydra, Kerberos, Nix, and Styx as a vivid planetary system.
The Main Event
One of the first jobs planetary scientists do when looking at a new space rock is count the number of craters on its surface. Based on the probability of a meteor strike, they can use these craters to calculate the surface’s age. Sputnik Planum—the western lobe of Pluto’s big heart—is an ice plain the size of Texas, and it has zero craters. “None. Not even tiny ones,” says Alan Stern, planetary scientist at the Southwest Research Institute in Colorado and principal investigator for the New Horizons mission. “That means it was born yesterday in geologic time span.”
That basic observation changes everything, because it implies that Pluto has active geology. The world has other features seemingly created by internal movements. “If you look at the surface of Pluto, the range of landforms is astounding,” says Stern: mountains, hillocks, scarps, glaciers, and canyons of different sizes, shapes, orientations, and ages.
It is important to note that what scientists call mountains are not exactly like the rocky structures commonly seen as desktop backgrounds here on Earth. “They are literally icebergs that poke out of the surface,” says Hal Weaver, NASA’s New Horizons project scientist, based at Johns Hopkins University Applied Physics Laboratory in Maryland. Icebergs most likely made out of water.
If you look closely at the mountains near Sputnik Planum’s northwestern border, you can see what look like glaciers carving around these icy peaks. These glaciers are made from frozen nitrogen, which behaves like water ice on Earth because Pluto’s surface temperature is around -395˚F.
These clues and others suggest Pluto has active geology, but nobody knows how. Conventionally, scientists understand tectonics in terms of tidal forces—the gravity from one celestial body tugging at the innards of its neighbor. “But Pluto is out there by itself, there’s no way to generate energy that way,” says Stern. Charon doesn’t count, because it and Pluto orbit each other in such a way that the tidal forces they exert on each other are essentially nil. “Somehow our understanding of the engines that run the insides of a small planet is wrong.”
Of course, scientists have other theories. One suggests Pluto’s bubble guts could have come from residual heat left over from when it formed. As the space rocks that eventually became the dwarf planet collided and collapsed into their current form, the gravitational energy pulling them together turned into heat. “But all the theories say that heat should have dissipated by now,” says Weaver. Another theory postulates that radioactive materials squished inside the planet during formation could be powering some of the activity. “But those radioactive decay rates only last for about 26 million years, maybe 100 million, but definitely not 4.6 billion,” he says.
Not all of Pluto’s secrets are buried below its surface. An atmospheric haze extends almost 100 miles above the world’s terrain. Structurally and chemically, it’s different from the nitrogen Pluto continually, mysteriously spews into space. This stuff is made from relatively large particles—caused by chemical reactions that cause nitrogen molecules to clump together. These deflect light in such a way that Pluto has a blue halo. Or, if you’re standing on the surface, a blue sky.
The haze is thin—Pluto’s surface pressure is around 10 microbars, compared to a viscous 1,013,250 microbars on Earth—and apparently windy. Looking closely at the haze, scientists have found what look like waves. These happen when denser, lower layers of atmosphere get being pushed against thinner, higher layers, or vice versa. (Streaks on Pluto’s surface also indicate some sort of blowing.)
All in the Family
Pluto is relatively tiny, but packs clout as the center of its own planetary system. Or maybe it’s the costar of a two-planet system. That’s because Charon is so large it doesn’t actually orbit Pluto. Instead, their barycenter is a point in space between the two (but closer to Pluto). This ambiguity doesn’t bother Stern. “Charon is Pluto’s moon, and it’s also a dwarf planet, and having this dual relationship is very common in science.”
Despite their proximity and shared history (Pluto and Charon were probably formed in the same series of collisions), the two worlds look quite different. Where Pluto is covered with colorful ices, Charon is relatively monochrome. Its dark polar region, nicknamed Mordor, is the exception. “The color looks similar to the equatorial regions on Pluto,” says Weaver. There’s probably a connection. Pluto is losing a lot of its nitrogen atmosphere into space. Charon’s gravity could be pulling some of it in, where it condenses over the cold polar regions and through solar interactions becomes dark, organic matter… In the land of Mordor where shadows lie…
Charon also has a giant chasm at least 1,000 miles long—though it’s possible the rift goes all the way around the moon. “It is many miles deep in places, and dwarfs the Grand Canyon,” says Weaver. The canyon is also a topographic boundary separating the rocky, mountainous “northern” hemisphere from the icy, smooth “southern” hemisphere. Called Vulcan Planum, the southern half was probably smoothed over by cryovulcanism—ice volcanoes. But like on Pluto, scientists are still at a loss for what could be causing Charon’s tectonics.
Orbiting Charon and Pluto are Styx, Nix, Hydra, and Kerberos. New Horizons managed to snap some pictures of these lumpy satellites, but so far details about what they are made of, and whether they are inhabited by little princes (kidding), have been scant. Most provocatively, the new paper describes Nix and Hydra as being much lighter than expected, which indicates that both are covered with relatively clean water ice.
This is pretty unexpected, because one would expect that that ice to be sullied with dust and meteoric dings after several billion years whipping through the Kuiper Belt.
What’s to Come
Nobody is promising to solve any of these mysteries, but consider the fact that this paper’s findings represent less than 5 percent of New Horizons’ data—the amount that had been downloaded by the end of July. “And that’s not even the best 5 percent,” says Stern. “We’ll submit five more papers by the end of next month. This is going to turn into a tidal wave of publications.” Currently, Stern and his team have about 20 percent of New Horizons’ total data.
Three months ago, Pluto and its satellites were pixelated blobs at the edge of the solar system. Of course, Stern would argue that Pluto, Charon, and the gang are only a halfway point—the gatekeepers to the Kuiper Belt. He would also argue that their mysterious geology and atmospheric emissions make them unique from rocks like Mars and Venus, spherical asteroids like Ceres, or even superficially similar moons like Neptune’s Triton. “If you stand way back and look at their features, this is the first exploration of a new class of planets,” he says.
If Stern is right, New Horizons is headed for another of these new Kuiper Belt planets. Called 2014 MU69, the probe should reach it by 2016. Hopefully by then somebody comes up with a less confusing name for it. After all, it only took them ten years to turn Pluto from pixels into a planet.
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